Given the increasing prevalence of obesity and fact that many adults have a strong preference to lose weight without attending weekly treatment meetings, there is need to develop effective alternatives to behavioral lifestyle interventions requiring less face-to-face contact. The Internet offers exciting opportunities to deliver behavior change interventions that minimize face-to-face interaction. We have recently developed and tested an Internet behavioral weight loss program compared with an Internet educational program in a randomized trial and found the behavioral program produced significantly better weight losses (4.1 kg) at 6 months. Our study clearly establishes the potential for using the Internet to deliver alternative treatment programs; however, treatment efficacy research is needed to further develop an Internet approach that will promote longer-term weight loss. The objectives of the proposed study are I) to enhance our Internet program to develop a state of the art Internet Cognitive-Behavior Therapy (I-CBT) program for obesity treatment; and 2) to conduct a randomized trial comparing the enhanced program with a Minimal CBT program also delivered via the Internet. We propose to recruit 100 overweight adults and randomly assign them to Enhanced Internet CBT or Minimal Internet CBT programs. The Minimal I-CBT condition will be given links to weight loss websites, weekly structured cognitive-behavioral lessons for weight loss, weekly prompting, and an on-line bulletin board. The Enhanced I-CBT program will have these same features plus weekly on-line group therapy sessions, computer-aided self-monitoring diaries, and weekly individual e-mail feedback from a therapist. The primary outcome is weight loss from 0-12 months. Secondary outcomes will examine patterns of weight change and changes in waist, diet, physical activity, and social support. The proposed research has significant implications for expanding the audience served by obesity treatment program by using the Internet. This study utilizes an innovative approach and extends our programmatic research on the development of a cognitive-behavioral Internet treatment for obesity.